Separation system using heat of compression

ABSTRACT

The present invention relates to a subsea system, where the subsea system comprises a separator ( 1 ) with an inlet line ( 2 ) and an outlet line ( 3 ), a compression unit ( 4 ) with an inlet line ( 5 ) and an outlet line ( 6 ) and a heat transfer unit ( 7 ), where the outlet line ( 6 ) of the compression unit ( 4 ) is guided into the heat transfer unit ( 7 ), the heat transfer unit ( 7 ) being connected to the inlet line ( 2 ) of the separator ( 1 ). According to the invention the heat transfer unit ( 7 ) is used for transferring heat between at least a part of a fluid in the inlet line ( 2 ) of the separator ( 1 ) and at least a part of a fluid in the outlet line ( 6 ) of the compression unit ( 4 ).

The present invention relates to a subsea system, and especially to a subsea system wherein at least some of the heat in the fluid flow resulting compression of the fluid flow is used to heat the fluid flow before it enters a separation stage. This subsea system is especially relevant for gas rich fluid flows or multiphase fluid flows.

BACKGROUND OF THE INVENTION

There are several systems known to provide separation of a well stream into different phases and thereafter transport the well stream to shore or a platform.

The present invention provides a device and method for providing a separation system with increased capacity in the case of a gas rich fluid stream or a multiphase fluid stream.

SUMMARY OF THE INVENTION

The invention is defined in the independent claim, while the dependent claims describe other embodiments of the invention.

According to the invention there is provided a subsea system comprising a separator with a fluid inlet line and at least one outlet line. The subsea system also comprises a compression unit for a gas rich fluid flow with an inlet line and an outlet line. The compression unit may be a pump, a multiphase pump, a compressor or other kind of element for increasing the pressure in the fluid and at the same time increasing the temperature in the fluid due to the compression. The system is further provided with a connection between the outlet line from the compression unit and the inlet line of the separator to provide heat transfer from at least a part of the fluid in the compression unit outlet line to the separator inlet line.

This provides a separation system that uses heat from gas compression or gas-liquid compression (also denoted multiphase pumping or wet gas compression) to increase the temperature in the process flow entering the separation station, so that the viscosity of the process flow is reduced and the separation efficiency of all involved phases therefore can be increased. The process fluids entering the separation station which are heated by the compressed fluids will have a reduced propensity to deposit wax or other substances onto the internal surfaces of the separators and any process equipment for produced water treatment.

An added benefit from this system is the reduction in temperature of the fluid in the compression unit outlet line as heat is transferred to the inlet line of the separator and therefore from the fluid in the compression unit outlet line.

The temperature increase associated with the compression of fluids will be reduced by means of heat transfer thus enabling further downstream processing of separated or non-separated process phases, where such processing is aided by the reduced temperature.

According to the invention, a possible embodiment provides a heat exchanger for heat transfer between the fluid in the compression unit outlet line and the fluid in the separator inlet line.

One may possibly use all the fluid in the compression outlet line for the heat transfer. This means guiding all the fluids at the compression outlet line through a heat exchanger. Another possibility is to provide a separator unit at the outlet of the compression unit to separate out a part of the fluid to be lead through a heat exchanger with the fluid at the inlet of the separator. There is also the possibility of having two heat exchangers in parallel arranged downstream of the compressor, one for each fluid phase out of the separator. Another possibility is to have a flow splitter at the outlet line of the compressor unit, to take only a part of the fluid through the heat exchanger. One may guide all the fluid from the compressor outlet line through the heat exchanger with the separator inlet line or only part of the fluid, and then possibly let the rest of the fluid bypass the heat exchanger. One possibility is to then combine the flows again after the heat exchanger, or another possibility is to lead one of the flows into another fluid line.

Another possibility is to take a part of the flow at the outlet from the compression unit and mix this with the well stream at the inlet of the separator. This part of the flow may be a part of a multiphase flow or a part of a phase divided flow. The temperature increase in the process fluid can be achieved by recirculation of and commingling with process fluid that has been compressed in a pump or compressor, thus avoiding the use of a heat exchanger. In other words, a part of the compressed process fluid can be bled off, its pressure relieved, and then it can be guided directly into the process stream to be heated. The bleed off may take place after or in a mixer to ensure an even distribution of phases in the two or more flows.

The compression unit in the form of a multiphase pump or compressor may, according to one embodiment, be placed after, or in other words, downstream of the separation station. By using a system according to the invention, one gets not only increased efficiency in the separator, but also cooling of the fluid flow after the compression unit. The separation station may comprise several stages and sub-processes. The multiphase pump or compressor can be placed between the separation stages or between process parts, according to the requirements of these stages and process parts.

Any gas in the process fluid can be separated from other phases after the pump or compressor, according to the requirements of the stages and process parts. By doing this, one may have a heat exchanger at the inlet of the separator with only one phase in the flow through the heat exchanger. Another possibility is to have one phase through the heat exchanger and at least a part of the flow not flowing through the heat exchanger bled down in pressure and introduced into the process flow to increase the temperature with mixing. Another possibility is to have this phase bypass the heat exchanger in a bypass line and be remixed with the split phase downstream of the heat exchanger.

Any gas in the process fluid can be intermediately separated from the other phases upstream of the compression unit.

The pump or compressor may alternatively be placed upstream of the separation station, thus improving separation efficiency through a temperature increase, and/or preventing wax or other temperature-influenced deposition on or inside the process equipment.

Any cooler between or after the separation stages or other process parts can be used to reduce the process fluid temperature, according to the requirements of equipment downstream of the cooler.

There is also the possibility of providing the system with additional sources for heating the fluid stream at the inlet of the separator. These may for instance be electric heating sources. Another possibility is to heat exchange the cooling fluid of the motor of the compression unit with the fluid at the inlet of the separator.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained with non-limiting embodiments with reference to the attached drawings where:

FIG. 1 is a schematic representation of one embodiment the invention;

FIG. 2 is a schematic representation of another embodiment of the invention; and

FIG. 3 is a schematic representation of yet another embodiment of he invention.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a representation of a first embodiment of the invention, only the elements relevant for the understanding of the invention being shown, as many additional elements in the system may exist. The subsea system shown in FIG. 1 comprises a separator 1 with an inlet line 2 and an outlet line 3. The inlet line 2 is connected to an upstream source which may, e.g., be the wellhead or another upstream subsea unit, as for instance a separator. There would normally be an additional outlet line from the separator 1, which is not shown in the drawings as it is not directly relevant for the invention. The subsea system also comprises a compression unit 4 with an inlet line 5 and an outlet line 6. The compression unit may be a compressor or a multiphase pump. The inlet line 5 of the compression unit may as indicated with the dotted line 10 be connected directly with the outlet line 3 of the separator 1. Another possibility is to have the inlet line 5 be connected to another fluid source. The outlet line 6 is guided into a heat transfer unit 7 which is connected to the inlet line 2 of the separator 1. This heat transfer unit 7 may be a heat exchanger or a mixer. In the case where the heat transfer unit is a heat exchanger 7, the fluid in the outlet line 6 of the compressor 4 may in one embodiment be guided through the heat exchanger 7. Exiting this heat exchanger, the fluid is cooled while heating the fluid in the inlet line 2 of the separator 1.

Another possibility is to provide a unit 8 in the form of a separator in the outlet line 6 downstream of the compression unit 4. This separator 8 would separate the outlet fluid in the outlet line 6 into two streams and possibly guide one of these streams through the heat exchanger 7 and the other stream into a bypass line 9. These streams may be connected again downstream or lead to different equipment subsea. Another possibility is to have the unit 8 be a splitter, splitting the fluid in the outlet line 6 into two or more streams, whereof one or several are guided through the heat exchanger 7.

Another possibility is to have the unit 8 split off a part of the fluid in the outlet line 6 and then introduce this fluid into a mixer 7 after the pressure is bled off to mix with the fluid in the inlet line 2 of the separator 1.

Also, the inlet line 2 to the separator may be divided, with one part leading through a heat exchanger and another part through a bypass.

FIG. 2 shows another embodiment of the invention. In this embodiment the separation process comprises a first separation stage and a second separation stage, in the form of primary and secondary separation, possibly arranged as a first separator 1 and a second separator 1A. A compression unit 12 is arranged downstream of the second separator 1A, and a compression unit 11 may possibly be arranged upstream of at least one of the separators, such as upstream of the first separator 1. The fluid exiting the second separator 1A is pressurized in the compression unit 12 and is then lead through a first heat exchanger 7 positioned between the first and second separators and then possibly through a second heat exchanger (not shown) positioned upstream of the first separator 1. The heat exchanger upstream of the first separator is positioned between the first separator and the optional compression unit 11. The possibility exists of using just one heat exchanger, the position of which being one of the above mentioned positions. The possibility also exists of using just one compression unit in this configuration.

Produced water from the first and second separation stages is guided into a produced water treatment unit 20. Oily reject from this treatment unit may be lead through a line 15 and introduced into the flow upstream of the first or second separation stage. The water to be re-injected into the well is lead out from the treatment unit 20 to a water reinjection pump 13. Part of the flow from the pump may be reintroduced through a line 16 back into the water treatment unit. In the embodiment shown in FIG. 3, produced water from the second separation stage is lead into a reject stream treatment unit 22, where it is treated along with the oily reject 15 from the water treatment unit 20. If the water from the reject stream treatment unit 22 is clean enough, it is directed to the reinjection pump 13; otherwise, it is lead back to the produced water treatment unit 20.

In the embodiments shown in FIGS. 2 and 3, the compression or multiphase pumping units 11, 12 are located at different steps in the process, in this case upstream of the first processing step and after the secondary separation step.

The compression unit 11 increases the stream temperature so that, e.g., the risk of wax precipitation in the oil and water treatment parts of the process is reduced. The temperature increase also enhances the separation efficiency, possibly allowing for a reduction in the size and weight of the separator vessels. Furthermore, with two-stage pumping, the size of the injection water pump 13 can be reduced. Heated injection water also has a lower viscosity, which may improve water permeation into the reservoir.

The advantage of multiphase compression in one or several stages with heat exchange is not only that the stream leaving the subsea process for further processing or transportation is cooled. Provided that the required injection water pressure is higher than the upstream process pressure, water pressure is available for recirculation back into the produced water treatment process. Single step multiphase compression upstream of the separation process would not facilitate this.

With the arrangement shown in FIG. 2, the temperature of the stream 14 is maximized. Also, since water is removed from the stream 14, the gas volume fraction into the pump or compression unit 12 is maximized, thus increasing the temperature out of the compression unit 12. In addition, gas is included in the hot side of the heat exchanger, and this gas has a relatively high heat capacity at normal processing pressures.

A further arrangement, not shown in FIG. 1 or FIG. 2, would be to split the gas and oil stream from the pump or compression unit 12 and lead it either as separate streams of gas and liquid, or as split multiphase streams, to two or more heat exchangers.

Another variety of this arrangement, also not shown in FIG. 1 or FIG. 2, is to cool part of the stream from the pump or compression unit 12 with seawater, and not heat exchange this part with the process stream.

Another variety of this arrangement, also not shown in FIG. 1 or FIG. 2, is to provide a bypass line around each heat exchanger in order to control the fluid flow rate entering the heat exchanger and thus optimize the amount of heat transferred in each device. The heat exchangers could also be arranged in parallel or in series. In FIG. 2 the downstream processing may be a cooling unit for precipitation of wax out of oil, so that a pipeline will not be clogged with wax as the oil cools. The heat exchange aids a downstream process like this. To prevent top-of-the-line corrosion, the heat exchange could also be part of a cooling sequence to condense water from the gas phase, to obtain controlled mixing with a corrosion inhibited aqueous phase. In FIG. 2 the oily reject stream 15 from the produced water treatment unit 20 may be recombined with the process stream up- or downstream of each separation stage.

The invention has now been explained with reference to the attached drawings and embodiments. Alterations and modifications may be made to these embodiments that are within the scope of the invention as defined in the attached claims. The possibility of combining features from the different embodiments to another embodiment of the invention also exists. 

1. A subsea system comprising: a separator having an inlet line and at least one outlet line; a compression unit having an inlet line and an outlet line; and a heat transfer unit for transferring heat between at least a part of a fluid in the inlet line of the separator and at least a part of a fluid in the outlet line from of the compression unit; wherein the outlet line of the compression unit is guided into the heat transfer unit and the heat transfer unit is connected to the inlet line of the separator.
 2. The subsea system according to claim 1, wherein the compression unit is a multiphase pump.
 3. The subsea system according to claim 1, wherein the compression unit is a compressor.
 4. The subsea system according to claim 1, wherein the heat transfer unit is a heat exchanger.
 5. The subsea system according to claim 1, further comprising a splitter which is positioned downstream of the compression unit to split out a part of the fluid in the outlet line of the compression unit and guide said part of the fluid into the heat transfer unit to mix with the fluid in the inlet line upstream of the separator, the heat transfer unit comprising a mixer.
 6. The subsea system according to claim 1, wherein the inlet line of the compression unit is connected to the outlet line of the separator.
 7. The subsea system according to claim 1, wherein the outlet line of the compression unit is connected to a downstream wax precipitation unit.
 8. The subsea system according to claim 1, wherein the heat transfer unit is provided with an additional heat source.
 9. The subsea system according to claim 1, wherein the system comprises at least a second compression unit arranged upstream of the separator.
 10. The subsea system according to claim 1, wherein the system comprises at least two separation stages which each comprise a corresponding first or second separator.
 11. The subsea system according to claim 10, wherein the heat transfer unit is a heat exchanger which is arranged downstream of the compression unit and upstream of at least the second separator to heat exchange a process fluid upstream of the second separator.
 12. The subsea system according to claim 11, wherein the system comprises a second heat exchanger which is arranged upstream of the first separator.
 13. The subsea system according to claim 12, wherein the subsea system comprises an additional separator upstream of the heat exchanger and downstream of the compression unit, such that the heat exchange takes place in parallel with split phases or one phase is bypassing the heat exchanger or mixed into a process fluid.
 14. The subsea system according to claim 1, wherein the subsea system comprises an additional separator upstream of the heat exchanger and a bypass line for at least one phase which extends around the heat exchanger. 